Grid-connected photovoltaic installations are formed by an array of photovoltaic panels (also called photovoltaic generator) and an electronic DC/AC converter (hereinafter converter), also called inverter, which prepares the energy produced by the photovoltaic panels and injects it into the electrical grid, where DC is direct current and AC is alternating current.
FIG. 1a depicts what is understood as direct current voltage VDC of a converter for connecting to single-phase alternating current voltage VAC of the state of the art. FIG. 1b depicts what is understood as direct current voltage in a converter for connecting to three-phase alternating current voltage VAC of the state of the art. If the converter is connected directly to the photovoltaic panel, the direct current voltage VDC and the voltage of the photovoltaic panel will be the same. If the converter is connected to the photovoltaic panel by means of, for example, a DC/DC converter, the direct current voltage and the voltage of the photovoltaic panel will be different.
It is thus established that the value of the minimum direct current voltage is determined by the conversion structure of the converter used and by the value of the alternating current voltage (i.e., voltage of the electrical grid in grid-connected photovoltaic installations).
On the other hand, the value of the maximum direct current voltage that the converter can withstand is determined by the characteristics of the components used in the converter, the most critical elements generally being the capacitor used to stabilize the direct current voltage and the switching devices (e.g., transistors and/or diodes).
Transistors used in converters usually depend on converter topology, on the direct current voltage and on the power of the converter, and the most widely used transistors are MOSFETs (Metal-oxide-semiconductor Field-effect transistor) and IGBTs (Insulated Gate Bipolar Transistor). However, the breakdown voltages of these switching devices do not adapt to the specific needs of each converter but rather follow standard values such as 600 V, 1200 V and 1700 V, for example.
Photovoltaic installations require converters with a broad direct current voltage range that allows working in the range comprised between the maximum power point voltage (Vmpp) of the photovoltaic generator at maximum ambient temperature, and the open circuit voltage (Voc) at minimum ambient temperature. The value of these voltages will depend on the photovoltaic panel configuration used (number of panels in series), on the ambient temperature and on the technology thereof. FIG. 1c shows two power (W)-voltage (V) curves of a photovoltaic generator of the state of the art for one and the same irradiance and for different ambient temperature values (P1=45° C. and P2=−10° C.).
On the other hand, the design trend for photovoltaic installations is to increase direct current voltage in order to reduce Joule-effect losses in the direct current wiring of the photovoltaic installation. Nevertheless, if the direct current voltage increases up to limit values, situations may arise in which the switching devices of the converter work close to their physical limits (breakdown voltage).
Furthermore, by increasing the direct current voltage, the converter can be connected to higher-voltage grids, increasing the power of the converter, making the photovoltaic installation more cost-effective because it is able to deliver more power to the grid with the same hardware.
Currently, the maximum direct current voltage limit of a conventional photovoltaic installation is set at 1000 V due to insulation levels of the photovoltaic panels. These direct current voltage levels could damage converters with 1200 V transistor-type switching devices with conventional switching techniques because overvoltages caused during switching operations of the transistors due to parasitic inductances (particularly while switching off the transistor) could exceed the 1200 V limit.
As can be seen in FIG. 2 of the state of the art, converters are formed from elementary switching cells, formed by a capacitor C for stabilizing the direct current voltage, two transistors T1 and T2 in series and an output inductor L connected at the intermediate point of attachment of the transistors T1 and T2. Said FIG. 2 also shows parasitic inductances L3 and L4 between the capacitor C and the transistors T1 and T2, and the internal parasitic inductances L1 and L2 of the transistors T1 and T2, which depend on the manufacture thereof.
Parasitic inductances L3 and L4 must be reduced as much as possible in the design stage because the intense current variations in said parasitic inductances L3 and L4 while switching on and switching off the transistors T1 and T2 cause overvoltages in the transistors T1 and T2.
As is known, the on/off state of transistors is controlled through the gate-emitter voltage. Control of this voltage is performed through a controlled voltage source or driver D1 and D2 (hereinafter, voltage source) and a gate resistor Rg1 and Rg2 connected between each voltage source D1 and D2 and the gate G1 and G2 of each transistor T1 and T2. FIG. 2 depicts, in addition to the gate G1 and G2, the emitter E1 and E2, as well as the collector CL1 and CL2 of the transistors T1 and T2. The voltage source is capable of imposing different voltage levels at its output depending on a control signal sent from the control unit of the converter, for example. An example of voltage sources from state of the art can be AVAGO hcpl-316j or hcpl-3120 of, among others.
The dynamics of the gate-emitter voltage establish the rate of current variation in the transistor (current derivative) while switching it on and switching it off and therefore the overvoltages occurring in parasitic inductances. Therefore, the lower the ohmic value of the gate resistor the greater the current derivative.
Furthermore, there is a relationship between the dynamics of said gate-emitter voltage and switching losses in transistors which translates into the idea that the higher the switch-on speed, the fewer the losses.
Overvoltages produced by current variations in parasitic inductances while switching on and switching off the transistors are more critical while switching them off, where current derivatives exceeding switch-on current derivatives are reached.
A solution of the state of the art for reducing switch-off overvoltages can be seen in FIG. 3 (Semikron-Application Note AN 7003 Markus Hermwille) and consists of using different gate resistors (RGT1, RGT2) in switch-on and switch-off. However, this solution has the drawback of the working conditions being set, and therefore if the overvoltages are to be greatly reduced, losses are high.
Another alternative used for not working with voltages close to the limit voltages of the components of the converter can be seen schematically in FIG. 4a. It consists of adding unrelated elements to the converter that reduce the direct current voltage of the photovoltaic generator from its open circuit voltage Voc to a safe value, after which the switching devices of the converter start to work. Thus, for example, part of the energy from the photovoltaic panels can be consumed in controlled resistors (also known as chopper).
However, the preceding solutions have several drawbacks. One of them is the increase in the cost of the photovoltaic installation, and another is that they do not allow progressively increasing the output power, as is required by certain grid regulations.
So another method used in the state of the art for not working with voltages close to the limit voltages of the components of the converter is the method shown in FIG. 4b, which consists of integrating an auxiliary switching element or a linear power supply source that reduces the voltage of the photovoltaic generator between the photovoltaic generator and the converter.
Nevertheless, although such solutions can be useful in low-power installations, they are not useful in medium- and high-power installations because the losses produced in the auxiliary switching element are very high.
Finally, another also known and very widely used alternative method consists of using transistors with a higher voltage range. However, these solutions also have drawbacks such as the increase in losses of the converter and a significant increase in economic cost.
This problems herein considered for photovoltaic installations are also present in other generating systems such as, for example, wind power systems of the type depicted in FIGS. 5.1a, 5.1b and storage systems of the type depicted in FIGS. 5.2a, 5.2b, and 5.2c. 
More specifically:
FIG. 5.1a shows a wind power topology formed by a wind-power generator of the type called back-to-back formed by two converters, a first machine-side AC/DC converter and a second grid-side DC/AC converter.
FIG. 5.1b shows the power-voltage curve of the generator characteristic of a wind-power generator. The working point is usually in the voltages providing maximum power (Vmpp) but under certain circumstances working with higher voltages (close to the Voc) may be interesting.
FIG. 5.2a shows a storage system formed by an array of electrochemical cells forming the battery (BAT) and a converter which can be a DC/AC or DC/DC converter for connecting to single-phase alternating current voltage of the state of the art.
FIG. 5.2b shows a storage system formed by an array of electrochemical cells forming the battery (BAT) and a converter which can be a DC/AC or DC/DC converter for connecting to three-phase alternating current voltage of the state of the art.
FIG. 5.2c shows a diagram showing the evolution of the current (I) and voltage (V) in different operating stages of the battery (discharge, constant current charging, constant voltage charging, float, equalization). As can be seen, during the equalization stage the voltage of the array of electrochemical cells forming the battery (BAT) is higher than in the usual operating stages (discharge, constant current charging, constant voltage charging, float).
Therefore, a control system for any type of converter (DC/DC or DC/AC or AC/DC) which further has switching devices working with direct current voltages close to the breakdown voltages is necessary in the state of the art.